Essential Oils and Their Constituents as Anticancer Agents: A Mechanistic View

Abstract

Exploring natural plant products as an option to find new chemical entities as anticancer agents is one of the fastest growing areas of research. Recently, in the last decade, essential oils (EOs) have been under study for their use in cancer therapy and the present review is an attempt to collect and document the available studies indicating EOs and their constituents as anticancer agents. This review enlists nearly 130 studies of EOs from various plant species and their constituents that have been studied so far for their anticancer potential and these studies have been classified as in vitro and in vivo studies for EOs and their constituents. This review also highlights in-depth various mechanisms of action of different EOs and their constituents reported in the treatment strategies for different types of cancer. The current review indicates that EOs and their constituents act by multiple pathways and mechanisms involving apoptosis, cell cycle arrest, antimetastatic and antiangiogenic, increased levels of reactive oxygen and nitrogen species (ROS/RNS), DNA repair modulation, and others to demonstrate their antiproliferative activity in the cancer cell. The effect of EOs and their constituents on tumour suppressor proteins (p53 and Akt), transcription factors (NF-κB and AP-1), MAPK-pathway, and detoxification enzymes like SOD, catalase, glutathione peroxidase, and glutathione reductase has also been discussed.

1. Introduction

Cancer has emerged as one of the most alarming diseases in the last few decades throughout the world. It is a multifactorial disease contributing towards uncontrolled growth and invasion of the abnormal cells leading to the formation of tumour. The steep rise in the number of cancer cases may be attributed to the change in food habits, use of tobacco and alcohol, chronic infections, exposure to harmful radiations and chemicals, or more widely due to change in lifestyle and environmental pollution . International Agency for Research on Cancer (IARC) reported that there are approximately 12 million cancer cases and these have accounted for 7.6 million deaths (around 13% of all deaths) in the year 2008 . The recent estimates reveal that the number of new cancer cases and cancer-related deaths has increased by 11% and 7.9, respectively, in the year 2012 as compared to 2008. Further, the developing countries have half the number of cancer incidence cases compared to the developed countries. In India, 0.979 million cancer cases were reported in the year 2010 which is expected to increase to 1.148 million by 2020. The mortality rate among cancer patients is very high. The problem is more serious in economically less developed countries due to the lack of diagnostic techniques, standard methods of treatment, and higher cost of the treatment. People in scientific field are currently overcoming these problems with the use of synthetic drugs. These drugs are designed to specifically target rapidly growing and dividing cells of various tumours. But, these synthetic drugs also affect rapidly dividing normal cells in our body leading to certain other major irreversible side effects. Chemotherapy used in cancer treatment has been reported to induce multidrug resistance . The high cost, increasing drug resistance, and side effects of current therapeutic approaches are forcing the scientists to explore alternative medicines, the traditional medicine, as an option to find new chemical entities for treatment of cancer.

Among the alternative traditional approaches, various plant products classified as alkaloids, saponins, triterpenes, glycosides, and polyphenols among others have shown very promising anticancer properties in both in vitro and in vivo. There are more than one thousand plants which have been reported to possess significant anticancer properties. Vincristine, vinblastine, colchicine, ellipticine, lepachol, taxol, podophyllotoxin, camptothecin, irinotecan, etoposide, and paclitaxel are classical examples of plant-derived compounds which are found to have wide applications in cancer therapeutics. The plant-derived products are expected to induce lesser side effects compared to synthetic drugs. Among plant derived compounds, essential oils (EOs) from aromatic plants have been reported to possess anticancer properties . EOs have also been reported to improve the quality of life of the cancer patients by lowering the level of their agony . EOs-mediated therapy cannot be a substitute to the standard chemotherapy and radiotherapy but can be used in combination with cancer therapy to decrease the side effects of the drugs. Hence, EOs can be used for improving the health of the cancer patients and as a source of novel anticancer compounds. In the last two decades, a number of researches are exploring anticancer potential of EOs and their components in vitro and in vivo models. Recently, Bhalla et al. reviewed EOs as anticancer agents limiting to the recent literature and a short mechanism(s) of action . However, the current review is a comprehensive one, enlisting nearly 130 studies of EOs from various plant species and their constituents that have been studied so far for their anticancer potential. The studies have been classified as in vitro and in vivo for EOs and their constituents. The current review also highlights in-depth various mechanisms of action of different EOs reported in the treatment strategies for different cancers.